Lab 3: Polygon Meshes

Code for this lab found, however it is recommended you clone the whole repository.

Task

1. Understand geometry in example file
2. Creating a 3D geometry
3. Implementing the geometry
4. Changing the camera view
5. (Extension) Load a 3D polygonal mesh from a file

Source code found and adapted from Karia Rodriguez

Journey

GL_ARRAY_BUFFERS GL_TRIANGLES

3 vertices 3 colours

GL_ARRAY_BUFFERS GL_TRIANGLES

12 vertices 3 colours

3. Implementing the geometry

I first started by grabbing a pen and piece of paper, drawing the current triangle I had written during lab 2. I had trouble picturing where the triangles were, so I moved them to both start drawing at (0,0) in their respective viewports. The triangles were now at a good starting point for me to visualise.

Starting at the bottom left corner I labelled the coordinates on the triangle, getting (0,0), (1,0), (0,1) for all three vertices. I drew the connecting edges on the paper to help visualisation. From here I marked the midpoint of the triangle, drew 3 more lines to simulate the additional triangles.

Using the formulae from the provided file I was able to generate the 3 vectors needed for each of the 4 triangles. This became easier as I moved the hard-coded values to variables, with s being the base length, z being 0, and h/H being the formulae.

    GLfloat s = .7;
    GLfloat z = 0.0;
    GLfloat H = ((sqrt(3) * s) / 2);
    GLfloat h = ((sqrt(6) * s) / 3);

The resulting vectors became

    static const GLfloat g_vertex_buffer_data[] = {
            //Base Triangle
            z, z, z,
            s, z, z,
            s / 2, h, z,
            //Triangle 1
            z, z, z,
            s / 2, h / 3, H,
            s / 2, h, z,
            //Triangle 2
            s, z, z,
            s / 2, h / 3, H,
            s / 2, h, z,
            //Triangle 3
            z, z, z,
            s / 2, h / 3, H,
            s, z, z,
    };

Resulting in

4. Changing the camera view To change the camera I edited the values in the camera matrix:

        glm::mat4 View = glm::lookAt(
                glm::vec3(s/2, H/3, s*3), // Camera is at (0,0,-1), in World Space
                glm::vec3(0, 0, 0), // and looks at the origin
                glm::vec3(0, 1, 0)  // Head is up (set to 0,-1,0 to look upside-down)
        );

I settled on the above vec as it meant we could see the pyramid head on.

5. Load a 3d polygonal mesh from a file

For the final task I imported the 3d file mesh. This required me to implement a new Viewer, FileShape.cpp. To get this to work I copied the code from the tutorial and abstracted the path to my Paths.h.

To make sure I could access both viewers I implemented a boolean condition:

int main(int argc, char* argv[]) {
    bool triangle = argc <= 1;
    
    ...
    
   if(triangle){
        t1 = new Triangle(width, height * 2, 0, 0, vertexbuffer, colorbuffer);
        t2 = new Triangle(width, height * 2, width, 0, vertexbuffer, colorbuffer);
    } else {
        t1 = new FileShape(width, height * 2, 0, 0, vertexbuffer, colorbuffer, sphere_asset);
        t2 = new FileShape(width, height * 2, width, 0, vertexbuffer, colorbuffer, sphere_asset);
    }

If there was any argument to the compiled binary the program uses the file - not the most elegant solution but it works.

Repo cleanup

I noticed that my folder structure was messy - I had to copy all the common code and libraries to each new lab. Therefore in this lab I performed an extensive refactor of the code and directory structure. I managed to extract the common libraries and shaders into a parent directory called libraries. This allowed me to have a cleaner, smaller repo, and keeps it DRY (Don’t Repeat Yourself).

This required me to edit the CMake files to allow for a parent libraries folder. It was easier than I had expected, the majority of what needed to be changed was the pathing inside each CMakeList. Changing these to relative paths (i.e ../../libraries/common) worked well.

I also extracted hardcoded paths to the shaders to a common file, Paths.h. This gives the ability to setup this project for any person by just replacing the absolute path in Paths.h only. I wrote a convience function in a Makefile that uses sed to replace the absolute path with the directory that the Makefile is in.

Final Product

Input:

make lab3
...
make lab3File

Output:

Conclusion

I found the introduction into 3D vectors difficult and it required a few sessions to get the program to work as expected. From the tutorial I gained experience in using vectors and visualising in a 3D space. I feel more confident working with OpenGL primitives and shaders, with hopefully more experience to come.

Sources

Stack Overflow

Tutorials Point

University of Brighton intranet/lectures